Shepard, who files his taxes separately from his wife, declined to comment to the newspaper.

A spokesperson for her re-election campaign, Meira Bernstein, told the Kansas City Star that McCaskill "does not make decisions on public policy based on what's best for her husband; she makes decisions based on what is best for the people of Missouri. That's why she voted against the recent tax bill that primarily benefited large corporations and the wealthy. And it's why she supported the Stop Tax Haven Abuse Act."

The bill, which did not pass, said that tax havens are "engaged in economic warfare against the United States, and honest, hardworking Americans."